1. Field of the Invention
This invention relates to a process for the production of low-chromium protein hydrolyzates.
2. Statement of Related Art
The fate of chromium shavings, which accumulate as waste products in the production of chrome-tanned leather, is a growing problem in the leather industry. The reason for this is that the presence of the chromium waste in dumps involves the risk of contamination of the ground water by Cr(III) and--after possible oxidation--even by Cr(VI). According to data provided by Taylor et al., 54,000 tonnes of chromium shavings accumulate annually in the USA alone (J. Am. Leather Chem. Assoc. 1990, 85, 264).
Boiling with alkali, usually lime or magnesium oxide, is one of the oldest methods for detanning chrome leather waste, cf. for example U.S. Pat. No. 4,100,154. According to the teaching of this patent, the alkaline hydrolysis of chrome leather is carried out at temperatures above about 90.degree. C. in the presence of calcium oxide or calcium hydroxide. By increasing the reaction temperature, which can be achieved for example by carrying out the reaction in a pressure reactor, the hydrolytic degradation rate can be further increased. According to this document, chromium is removed to residues "below 5 ppm", although the exact concentrations of collagen hydrolyzate in the solutions and hence the reference value for the Cr content are not disclosed.
DE-A-1 000 388 describes a process for the production of protein degradation products in which the hydrolysis of chrome leather is carried out with water or aqueous ammonia at elevated temperature and pressure.
The desire to avoid the drastic reaction conditions of the conventional protein hydrolysis processes mentioned above has resulted in the development of enzymatic hydrolysis processes. For example, German patent application DE 22 52 281 describes a process for the hydrolysis of skin fragments using neutral or alkaline proteases which are capable of decomposing collagen. Leather fragments inter alia may be used as the skin fragments. Before the enzymatic treatment, the protein of the skin fragment is denatured by heating with water. The enzymatic hydrolysis is subsequently carried out at moderate temperatures of 20.degree. to 70.degree. C. and is typically followed by thermal denaturing of the enzyme and by filtration of the crude product obtained.
In a more recent publication, Heideman et al. discuss in detail possible methods for treating chromium shavings. So far as the hydrolysis of chromium shavings with proteases is concerned, they come to the conclusion that the preliminary denaturing of the shavings plays a critical role. They obtained optimal results when they first digested chromium shavings with 10% of lime for about 20 minutes and then treated the shavings with protease (cf. Das Leder 1991, 42, 133-143).
It has only recently been shown by Taylor et al. that preliminary denaturing is not absolutely essential in the enzymatic hydrolysis of chrome leather waste. In their method, the protein present in the waste is treated with special mixtures of alkali metal and alkaline earth metal compounds and alkalases at moderate temperatures without preliminary denaturing. The authors reported on their method at the 85th Annual Conference of the ALCA (J. Am. Leather Chem. Soc. 1990, 85, 262). In addition, this method is the subject of U.S. Pat. No. 5,094,946. Furthermore, the effectiveness of the method was confirmed in practical tests carried out at the Danish tannery Svendborg Fingarveri ("Bio Times" [quarterly magazine of the Novo Nordisk Company] 1990, 5, No. 1, pages 4-5). However, the method developed by Taylor et al. has the disadvantage that the aqueous concentrates thus prepared are not stable in storage, but instead become cloudy over a period of time.